Acquisition of seismic data through the use of seismic surveying techniques provides important data that is helpful in a number of different industries and contexts. For instance, seismic surveys may be utilized to create images of subsurface geological structures. These images may then be used to determine, among other applications, optimum places to drill for oil and gas and to plan and monitor enhanced resource recovery programs. Seismic surveys may also be used in a variety of additional contexts outside of oil exploration such as, for example, locating subterranean water and planning road construction.
A seismic survey is normally conducted by placing an array of seismic acquisitions units or vibration sensors (accelerometers or velocity sensors called “geophones”) on the ground, typically a line or in a grid of rectangular or other geometry. Vibrations (often referred to as “source events”) are created, such as by explosives, a mechanical device such as a vibrating energy source or weight drop. These vibrations propagate through the earth, taking various paths, refracting and reflecting from discontinuities in the subsurface. For instance, the seismic energy may propagate into the earth and be reflected and/or refracted by subsurface seismic structures (e.g., interfaces between subsurface lithologic or fluid layers characterized by different elastic properties). In turn, the refracted and reflected seismic energy may be detected by the array of vibration sensors deployed in connection with the seismic survey. In turn, signals at the sensors may be amplified and digitized, by separate electronics and/or internally in the seismic acquisition units. In other contexts, the survey may be performed passively by recording natural vibrations in the earth. In any regard, the data from the sensors is eventually processed to create the desired information about the subsurface geological structures in the area being surveyed.
Recently, seismic survey systems that forgo traditional cabling to provide data collection have been proposed with recognition of the benefits of providing cableless seismic acquisition units in a seismic survey (e.g., associated with the elimination of certain problems associated with deploying, maintaining, and collecting cables from a survey area). However, such cableless units present additional considerations in maximizing the benefit of the use of the cableless units. For example, the serviceable deployment time of the units can be reduced compared to cabled units as each cableless unit must generally rely upon a local power source (e.g., a battery or the like). In turn, the battery life is often a limiting factor in the serviceable deployment time of a cableless acquisition system. This limitation on serviceable deployment time applies to both cableless systems that utilize telemetry for data read out as well as nodal systems that store data locally at each acquisition unit during the course of the seismic survey.
Furthermore, it is important to precisely know the location of each cableless unit in a cableless system as well as synchronize the timing systems of the various cableless units to produce reliable information regarding the subsurface geological structure(s) of interest. However, determining the location of each cableless unit and synchronizing the timing systems of the units can place additional demands on the local power sources and further limit serviceable deployment time. Furthermore, labor intensive and costly surveying techniques are often required to precisely locate the various units in a survey (e.g., including surveyors employing hand-held GNSS (e.g., GPS) modules and other equipment to precisely locate each unit during deployment of the units in a field in any appropriate pre-defined pattern).
Some traditional systems utilize location determination units (e.g., GPS modules having GPS receivers, processors for processing received GPS signals, memory, etc.) on board each cableless seismic acquisition unit that are each operable to resolve each cableless unit's location and provide a coordinated reference to Coordinated Universal Time (UTC). Such location determination and timing synchronization may occur at the deployment of units and/or throughout the course of the survey. However, the use of GPS modules at the cableless units requires significant amounts of power, thus quickly degrading the serviceable deployment time of acquisition unit. Moreover, while attempts have been made to reduce the power requirement for such GPS modules (e.g., by duty cycling the GPS module to periodically activate, acquire location and timing information, and deactivate), the need to include a GPS module each acquisition module can still be cost prohibitive—this may especially be the case for very large arrays with thousands, tens of thousands, or, in anticipated applications, hundreds of thousands or more than a million units. Still further, the GPS modules are typically not able to resolve locations to any greater than about five meter accuracy. As a result of such inferior location accuracy, traditional approaches still often require traditional surveying techniques. Even without on-board GPS modules, high channel (e.g., 1,000,000 or more) systems using traditional cableless units would be cost prohibitive.